This invention relates to magnetic resonance (MR) imaging apparatus.
Magnetic resonance is excited in MR active nuclei, such as protons of hydrogen atoms, by applying an r.f. excitation pulse in the presence of a powerful magnetic field. The resulting r.f. signals emitted by the nuclei, in the presence of suitable spatial coding of the magnetic field, provide data about the distribution of these nuclei and hence of tissue and fluids in the patient being imaged.
One way of achieving the high magnetic fields necessary to enable resonance to be excited is by using an electromagnet in the form of a coil, either resistive, or more usually superconducting. The patient is slid into the bore of such a magnet.
A typical arrangement is shown schematically in FIG. 1. A patient support 1 is moveable on runners 2 on a beam 3 into and out of the bore 4 of an electromagnet. Coils for producing the main magnetic field are contained in the casing 5 (not shown to scale) surrounding the bore 4, as are coils for producing magnetic field gradients for spatially encoding the part of the patient which lies within the excitation region.
For a given number of turns and current, the field strength along the axis of a coil decreases as the diameter of the coil is increased. This decrease of field could be offset by increasing the current through the coil, but with the penalty of increased cost of the magnet. Accordingly, there is always a conflict between desired minimum bore size and the cost of the magnet. It follows that care is always taken to utilise the full diameter inside the bore.
One constraint on the utilisation of the full diameter of the bore is the need for an r.f. excitation coil 7. For imaging the whole body, typically a body coil such as a birdcage coil would be used, and this must be located inside the bore 4 (rather than in the casing 5 surrounding the bore) since its effectiveness decreases the further it is positioned from the patient. In cases where part only of the body e.g. the head, is being imaged, other types of excitation coil may be used, but a coil such as a body coil is needed when it is desired to image the full length, or at least a substantial length, of the body.
One of the cases when it is necessary to image the full length, or at least a substantial length, of the body is in the case of continuous scanning, to which this invention is particularly applicable.
In a continuous scan, a patient is moved through the bore while image data is obtained from a relatively small volume of good quality magnetic field near to or at the centre of the bore. In principle, it is possible to scan a subject from head to toe.
Thus, in the case of continuous scan, the simplest arrangement is to use a whole body transmit coil 7 located in the bore of the machine, and to use this coil for receiving r.f. signals as well. There are, however, problems with this arrangement. Firstly, the r.f. coil 7 takes up significant space in the bore 4 of the magnet, and this restricts the possibility of shifting the patient laterally to optimise scanning the regions offset from the centre of the body. To scan such offset regions, it would be desirable to place them in the centre of the field. Another problem is that large whole body coils have unnecessarily large volumes of r.f. field with field leaking out from the magnet bore during transmission, and consequent excitation of regions of the body from which signal is not sought, causing aliasing. Also, the large r.f. coil is sensitive to unwanted regions of the body therefore causing additional coil noise and loading. Further, a whole body coil is poorly coupled to the body when receiving MR signals with consequent loss of signal-to-noise ratio.
It has been proposed to make the body coil 7 elliptical to get better coupling, but then this makes it more difficult to slide the patient through.
For receive purposes, surface coils (which may be flexible), and are placed on the surface of the body have been proposed, as have arrays of coils, but these are not suitable for transmit purposes. From a receive point of view, field sensitivity is not all that important, and can be corrected for. Transmitter pulses require simple and accurate fields if signal contrast is to be controlled, generating parts of a flip from components of an array.
The invention provides a magnetic resonance imaging apparatus comprising a magnet having a bore into which a patient support can be introduced, an r.f. coil for transmitting r.f. signals, wherein the r.f. coil is secured to the patient support so as to be moveable with the patient support along the bore, the r.f. coil extends part way around the axis of the bore in order to partly surround the patient while allowing access to the patient support, and at least a part of the r.f. coil being displaceable laterally with respect to the bore to permit imaging of different regions of the body.
This permits imaging to be carried out with the patient shifted laterally in the magnet bore as well as allowing better localisation of the r.f. field generated to be achieved.
The invention also provides a magnetic resonance imaging apparatus comprising a magnet having a bore into which a patient support can be introduced, an r.f. coil for transmitting r.f. signals, wherein the r.f. coil is secured to the patient support so as to be moveable with the patient support along the bore, the r.f. coil extends part way around the axis of the bore in order to partly surround the patient while allowing access to the patient support, the r.f. coil including an array of coils extending in a direction around the patient, and the width of one of the coils is adjustable in a lateral direction with respect to the bore.
Ways of carrying out the invention will now be described in detail, by way of example, with reference to the accompanying drawings, in which: